In recent years, the technical advance of a large scale integrated circuit (LSI) device is significant and, in particular, an improvement in the integration density is remarkable. With an improvement in the integration density, the size of the IC chip has been increased and the density of the wiring per IC chip has been increased. This has led to a remarkable increase in the heat build up per IC chip. For this reason, great importance has come to be placed on the heat radiating property of a material constituting a package on which IC chips are mounted. An alumina sintered body has hitherto found extensive use as the material of an IC chip substrate. With the heat conductivity of the alumina sintered body, however, it is becoming difficult to ensure a heat radiating property sufficient to cope with an increase in the heat buildup of the IC chip. Studies have been made also on the use of a beryllium oxide sintered body having a high heat conductivity as an alternative to the alumina sintered body. The beryllium oxide is however, difficult to handle due to the toxicity of the material per se and therefore can be used for a special application only.
On the other hand, an aluminum nitride sintered body has a heat conductivity (theoretical value: 320 W/m.multidot.K) comparable with that of the beryllium oxide sintered body, is nontoxic and has a high insulating property. Therefore, it has attracted special interest recently in the field of semiconductor industries and electronics including a high output laser, wherein the heat radiating property becomes an issue.
As described above, aluminum nitride is theoretically a material having a high heat conductivity and a high insulating property as a single crystal. Since, however, aluminum nitride is a material having a covalent bond character of 50%, the diffusion coefficient of the constituent elements is small. Further, the sinterability of the aluminum nitride powder is much lower than that of the aluminum oxide powder. Therefore, when a sintered body is produced from the aluminum nitride powder, the relative density (based on the theoretical density of aluminum nitride, that is, 3.26 g/cm.sup.3) of the aluminum nitride sintered body produced by powder molding followed by sintering is as low as 70 to 80% although it depends upon the sintering conditions, and the sintered body has a large amount of pores.
On the other hand, the heat conduction of the aluminum nitride sintered body having an insulating property occurs through the movement of phonons as pseudo-particles. For this reason, defects such as pores or impurities in the crystal lattice cause scattering of phonons, which brings about a lowering in the heat conductivity of the aluminum nitride sintered body.
Under these circumstances, various proposals have been made for the purpose of producing an aluminum nitride sintered body having a high conductivity.
In order to produce an aluminum nitride sintered body having a high conductivity, however, it is necessary to use a starting material having a high purity and to prevent the contamination with impurities in the manufacturing process. The resultant aluminum nitride sintered body is limited to a white or thinly colored sintered body, which renders the sintered body unusable in applications where the transmission of light becomes a problem. For this reason, the development of a colored aluminum nitride sintered body has been desired in such applications.
The present inventors have already proposed a highly heat conductive aluminum nitride sintered body having a light shielding property in Japanese Patent Laid-Open No. 124772/1990.
In the conventional light shielding aluminum nitride sintered body, the light shielding property is obtained by distributing a transition metal compound in the sintered body and allowing the light to be absorbed or diffused in the compound. No properties of the aluminum nitride sintered body, such as heat conductivity, can be sufficiently obtained depending upon the type of the transition metal added. This appears to derive from the fact that the transition metal compound in the aluminum nitride sintered body is not sufficiently controlled.
Accordingly, an object of the present invention is to eliminate the above-described problem and, at the same time, to provide an aluminum nitride sintered body stabilized in the properties such as light shielding property and heat conductivity and a process for producing the same.